Adsorption properties and mechanism of montmorillonite modified by two Gemini surfactants with different chain lengths for three benzotriazole emerging contaminants: Experimental and theoretical study

Two gemini surfactants propylbis (dodecyldimethyl) ammonium chloride (12-3-12) and propylbis (octadecyldimethyl) ammonium chloride (18-3-18) with different chain lengths were used to modify Ca- montmorillonite (Ca-Mt). XRD results showed that the arrangement of surfactants was mainly paraffin-type and with additional minor lateral-bilayer in the interlayer space of montmorillonite. Batch adsorption tests indicated that the modified organo-montmorillonite (G12-Mt and G18-Mt) had better removal effect on three benzotriazole emerging contaminants (ECs) than that of original Ca-Mt, and the sequence of adsorption capacity was 1-Hydroxybenzotriazole (OHBTA) > benzotriazole (BTA) >5-methylbenzotriazole (TTA). Molecular dynamics simulation was carried out to reveal the arrangement, conformation and behavior of Gemini surfactants in the interlayer of Mt. at atomic level. The results suggested that one of the two long alkyl chains was erected in a paraffin-type arrangement, the other long alkyl chain accompanying with the positively charged N head groups & the propyl spacer were parallel to the Mt. (001) plane and was fixed near the top and bottom surfaces with the arrangement of lateral-bilayer, which was agreement with the XRD data. BTA molecules located at the aggregate organic phase formed by the alkyl chain, while Cl- ions preferred to appear in the middle area of the interlayer of montmorillonite. Meanwhile, Multiwfn wavefunction program was applied to quantitatively analysis the molecular surface area in different electrostatic potential (ESP) intervals, the ESP minimum & maximum values on the three ECs molecular vdW surface, integrating with color-filled ESP planar maps and VMD program to insight the electrostatic interaction and magnitude order between Gemini surfactants modified Ca-Mt and three pollutants. The adsorption mechanisms of Gemini surfactants modified Mt. to three benzotriazole emerging contaminants involved electrostatic interaction and hydrophobic partitioning effect.

Automated summary

The study found that two gemini surfactants modified montmorillonite had better removal effect on benzotriazole emerging contaminants, involving electrostatic interaction and hydrophobic partitioning in the adsorption mechanism.